This invention relates to focused, electron-bombarded (FEB) detectors, particularly to FEB ion detectors.
Ion detectors are used to measure charged particle flux or current in a vacuum. When used in mass spectrometers, for example, ion detectors measure the flux of charged particles moving through an orthogonal magnetic field of a predetermined magnitude and bending according to their charge to mass ratios. Other applications include scanning electron microscopy; UV and X-ray spectroscopy; E-beam/X-ray lithography; field ion microscopy; and charged particle/photon imaging. Prior art mass spectrometers and other devices use channeltrons or stacks of microchannel plates (MCPs) as ion detectors.
A channeltron or channel multiplier is based on the continuous dynode electron multiplier concept first suggested by P. T. Farnsworth in U.S. Pat. No. 1,969,399. The channel multiplier consists of a hollow tube coated on the interior surface by a secondary electron emitting semiconductor layer. This layer emits secondary electrons in response to bombardment by electromagnetic radiation or particles such as electrons. Input and output electrodes are provided on each end of the tube to create a bias voltage which accelerates the emitted secondary electrons down the channel. Secondary electrons also strike the wall, releasing additional secondary electrons. The resulting amplification of the input photon or particle is called the device's gain.
MCPs operate on the same basic principles as channeltrons. A typical MCP is comprised of a million parallel channels 4-20 microns in diameter and 40-500 microns long. The channels are typically formed at a small angle of a few degrees relative to the normal MCP surface to ensure that ions generated at the tube anode cannot be accelerated down the channel but instead strike the channel wall near the back of the MCP. MCPs may be stacked to multiply the effect of their gains. When stacked, the channel angles of alternating plates are reversed, giving rise to the designation "Chevron" or "Z" stack.
One drawback of prior art ion detectors such as channeltrons and MCP stacks is their relatively large capacitance, which limits their bandwidth and response time. Another drawback of prior art ion detectors is their limited dynamic range. This second drawback is due, in part, to the high gains needed for certain applications. At high bias voltages, channeltrons and MCP stacks can become nonlinear in their output current responses to input currents.
Yet another drawback of current detectors is the need to operate the MCP stack or channeltron at high bias voltages. High bias voltages accelerate the degradation of the electron-emitting channel surfaces. In addition, operation of the devices under high biases requires the maintenance of a high vacuum to minimize the effect of ion feedback on the performance of the devices.
What is needed, therefore, is an ion detector that has increased dynamic range, bandwidth and shorter response time.